The invention relates to an apparatus for manufacturing tube--like preforms having a variable cross-section along their length for blow--molding hollow bodies from a thermoplastic material, which apparatus comprises a tubular die and a collar die (injection molding die) having a recess into which plastic material is injected when the collar die sits on the tubular die, the preform being produced upon lifting of the collar die of the tubular die and moving collar die to its predetermined position above the tubular die. The apparatus further comprises a control device for a drive displacing the collar die toward and away from the tubular die, and a control device for a drive responsible for changing the size of the exit opening of the tubular die during movement of the collar die away from the tubular die. The speed of the collar die increases from zero up during away movement of the collar die. The control devices are actuated upon start of lifting of the collar die and effect a displacement dependent control.
An apparatus of this type is disclosed in German patent 2,426,730. This apparatus functions according to a PRESSBLOWER process. The control devices of this apparatus are provided with switches which are actuated by the liftable collar die. The preform produced in this apparatus has a wall with a stepwise cross-section. The wall thickness of the preforms in different regions has a different configuration, and the change in cross-section of the preform is produced in relatively little time. This known apparatus is poorly adapted for producing preform for bellows. This apparatus is not suited for producing preforms for bellows that should have a substantially constant wall thickness despite having numerous large pleats. This requires practically not attainable large number of switches. It is also known, from German patent 2,426,736, to control switches of the control device of such an apparatus, which switches are arranged along the displacement path of the collar die, by an electronic prearranged program, If such a program is made time-dependent, then frequent changes in wall thickness, necessary for producing a preform for a bellows, cannot be effected with a necessary precision, because conformity between respective positions of the collar die cannot be achieved with sufficient precision.
Accordingly, the main object of the invention is an improved apparatus of the above-mentioned type which would make it possible to produce preforms for bellow, that is performs having a relatively high number of enlarged wall thickness changes per unit of length, with an increased precision.